The present invention relates to a new and improved method of, and apparatus for, the determination of the fault direction with respect to a measuring location at electrical lines or networks containing a series capacitor or capacitance.
The prior art is already acquainted with methods and apparatuses for fault direction determination with respect to a measuring location or site at electrical lines having series capacitance, wherein the phase angle of the difference of an equivalent or image line voltage signal U.sub.E and a line voltage signal U.sub.K at the measuring location in relation to a reference voltage U.sub.r signal are compared with a threshold or limit value, typically especially 90.degree.. Upon exceeding or falling below this threshold value there is derived a fault direction signal. As the line-image voltage signal there is employed the potential drop at an image or equivalent impedance to which there is applied a line current signal. In the relay trip or forward direction, considered with respect to the measuring location and determined by the positive direction of current flow, a line or network section is provided with a line impedance Z.sub.Lr and a current source having a source impedance Z.sub.Sr. The magnitude of the source impedance can be the same or greater than that of the line impedance.
The series capacitance or capacitor which is arranged in the current path of such circuit configurations serves, in known manner, to extensively compensate the inductive reactance of the line and is usually provided with a bridging spark gap or path which, in the event of short-circuit currents flowing through the capacitor, fires or arcs over above a certain threshold or limit by virtue of the corresponding potential drop across the capacitor and protects the latter against overloading. If there is presupposed that between the capacitor and a current source, which in the equivalent circuit diagram has a source impedance, there is present a line impedance, then while taking into account usual spark gap settings, it will be apparent that there is not insured for any reliable response of the spark gap when the magnitude of the source impedance with respect to that of the line section between the source and the capacitor is the same or greater. This is so because in conjunction with the line impedance the source impedance then provides a total impedance value which, under certain circumstances, does not sufficiently limit the short-circuit current to values adequate for response of the spark gap. Without bridging the capacitor by the low resistance arc of the ignited spark gap, there then appear, following a short-circuit near to the capacitor, at its rear side, i.e. the side facing away from the source feeding the short-circuit, transient oscillations having a frequency essentially determined by the capacitance of the capacitor and the inductances (line and source inductances) which are effective in the resonant circuit. These oscillations of the short-circuit current lead to faults or disturbances in the function of a directional or distance relay arranged, for instance, between the capacitor and the line. This relay should respond to short-circuits appearing at the line between the relay and the source, i.e. in the sense of triggering a fault cut-off, however in the event of faults at the rear side of the capacitor, as the same has been previously assumed, this relay should however block. The faulty operation, as has been found in practice, is attributable to the fact that notwithstanding a fault position at the rear of the relay, i.e. in the blocking area, there nontheless occurs a tripping or response of the relay. The reverse faulty operation, wherein namely in the event of a short-circuit at the line in the forward direction of the relay (i.e., that direction with respect to the relay site where the relay should respond and trip in the presence of short-circuits) the relay does not trip, does not really occur in practice due to transient current oscillations. The forward direction of a relay of standard design having a line-image voltage signal and a difference voltage signal, whose phase is monitored in relation to a reference voltage and compared with a threshold value or limit of typically 90.degree. (circular trip characteristic or region in the complex impedance plane), is determined by the positive flow direction at the line or in the image or equivalent impedance.